A known type of negative-pressure booster is disclosed in SAE 950760 and includes a housing in which is formed at least one pressure space and a movable wall provided in the housing for advancing and retreating movement with respect to the housing. The movable wall divides the pressure space inside the housing into a front chamber that is adapted to communicate with a negative-pressure source and a rear chamber that is adapted to selectively communicate with the front chamber or atmosphere. A power piston is connected to the movable wall, and an input member is provided inside the power piston for advancing and retreating movement with respect to the power piston. An air valve seat is provided in the power piston for advancing and retreating movement together with the input member. A negative-pressure valve seat is provided in the power piston. A control valve has an air sealing portion capable of contacting and separating from the air valve seat for isolating the rear chamber from the atmosphere by coming into contact with the air valve seat and for causing the rear chamber to communicate with the atmosphere as a result of being separated from the air valve seat. A negative-pressure sealing portion is capable of contacting and separating from the negative-pressure valve seat for isolating the rear chamber from the front chamber by coming into contact with the negative-pressure valve seat and for causing the rear chamber to communicate with the front chamber by being separated from the negative-pressure valve seat. An output member outputs outside the booster an advancing force of the power piston that is generated in association with movement of the movable wall and a reaction member transmits the advancing force of the power piston and an input that is applied to the input member to the output member. The reaction member also applies a reaction force corresponding in magnitude to the output of the output member to the input member to cause the input member to retreat. An actuator causes the rear chamber to communicate with the atmosphere by separating the air sealing portion from the air valve seat. The known negative-pressure booster is further provided with a detecting device for detecting the volume of separation between the air valve seat and the air sealing portion, and the actuator is caused to operate in accordance with the detection result of the detecting device.
However, the presence of the detecting device which detects the volume of separation between the air valve seat and the air sealing portion necessarily increases the cost and complexity of the negative-pressure booster.
In light of the foregoing, a need exists for a negative-pressure booster having a relatively simple construction while at the same time being relatively low in cost.